Its tens of thousands of big brown trucks, largely powered by diesel engines, will soon be joined by 50 Workhorse plug-in hybrid delivery vans.

And, UPS said, those new vans cost it no more than diesel versions with the same capacity.

That’s a big advance for trucks that plug in, and may well encourage fleet managers to take a closer look at trucks that plug in to provide part or all of their range.

In a press release issued in late February, UPS said the 50 plug-in electric delivery trucks it will deploy “will be comparable in acquisition cost to conventional-fueled trucks without any subsidies.”

That achievement is, it said, “an industry first that [breaks] a key barrier to large-scale fleet adoption.”

Indeed, it is the high cost of lithium-ion battery packs to provide adequate range in large, heavy delivery trucks that has proven to be the hurdle to wider adoption thus far.